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More on props and performance
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| burtim |
Posted on 04/21/08 - 7:36 PM
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OK, so I've tested my 2001 sport 13 and have some questions.
I get 30+ MPH @ 5000 RPM with 2 adults and full fuel. Well below the motors maximum RPM limit. Perfect for my fishing adventures on local lakes using the factory stock prop.
The problem is, with a full boat, max. RPM drops back to about 2200 RPM and forward speed becomes very low. I can't quite get on plane regardless of loading.
I really have two missions. One is a light boat for fishing that needs to go fast (well, 30 is fast enough). Another is a boat loaded down with stuff including 2-3 adults and kids for light cruising that is optimized for best speed.
I studied the equations and prop information on www.continuouswave.com and did a calculation. I should be getting high 20's with that motor. I don't think the performance should drop off anywhere near as much as I see it when I load the boat with an additional 250 lbs. or so if I optimise the power transfer. Am I wrong?
I have a 10-3/8" x 14 pitch prop now and have thought about purchasing a second prop if that would give me better performance for the full boat. If I keep the diameter and go to a 12 pitch prop. the RPM will increase at WOT. Will that actually result in increased HP and therefore speed in the loaded condition or, will I simply see the tachometer show higher RPM with the attendant waste of fuel?
I understand that if I leave the lower pitch on with a lighter load I run the risk of overrevving. I won't do that because I will put the appropriate prop on for the load.
I don't mind changing props depending on the need at the time. I pull the prop for trailering anyway.
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| Phil T |
Posted on 04/21/08 - 8:07 PM
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Burtim -
While you have given us most of the important information, there are a few elements missing.
Do you know what she runs like with just you and light gear with a full tank?
While I see from your personal page the boat has a Mercury outboard, what horsepower?
Where is the engine mounted in relation to the transom. In your photo, it is hard to see. Ideally, the anti-cavitation plate should be slightly above the keel of the boat. Where is yours? A less accurate method is bolt hole. On the top set of holes, which one has the bolt and how many empty ones are above the bolts?
1992 Outrage 17 I
2019 E-TEC 90, Viper 17 2+
2018 Load Rite Elite 18280096VT |
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| burtim |
Posted on 04/21/08 - 8:11 PM
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OK. It's a 40 HP factory intalled pakage. The anti-cavitation is just above the bottom of the hull.
It does more than 30 MPH and just above 5,200 RPM with me alone.
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| Tom W Clark |
Posted on 04/21/08 - 8:26 PM
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Burtim,
You have several options. If you have a Mercury 10-3/8" x 14" prop then I am assuming it is part #48-816706A40 which is the "large blade area" aluminum three blade for the 2001 40 HP two stroke. Correct me if I am wrong.
You could try the 10-1/4" x 14" aluminum version, part #48-73138A40 or drop down to either the 10-1/2" x 13" aluminum three blade, part #73136A40 or the 10-3/8" x 13" aluminum three blade, part # 816704A40.
A better choice than any of these would be a good stainless steel prop. Mercury offers the 10-1/2" x 13" three blade Vengeance, part #48-855858A 5 or you could go aftermarket and try a Stiletto Triad II in the 10-1/2" x 13" size, part #12313.
Do you know what altitude you are usually boating at? That may have something to do with the lackluster performance.
Edited by Tom W Clark on 04/21/08 - 8:27 PM |
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| moose |
Posted on 04/22/08 - 6:58 AM
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Burtim,
Tom W Clark is THE man for props. I'll only add that I am using a 13" ss prop on my classic 13 with 40 hp Nissan and is a good all around set up.
Mike
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| burtim |
Posted on 04/22/08 - 6:40 PM
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The altitude is only 1,000' MSL. I don't think that should be a big issue for us.
the prop is the aluminum 10-3/8" X 14 pitch, I don't know the exact part number. Anyhow it seems like you are suggesting, as I suspected, that I should lower the pitch. That should increase RPM's and HP delivered to the water right? I guess my basic question is still unanswered. Does insuring that the motor is turning at some value approaching 5,500-6,000 RPM at WOT also guarantee that all of the horsepower available is delivered for maximum forward thrust? I have been assuming that because the maximum RPM under load was limited to 2,500 RPM or so, I wasn't getting all of the power I can.
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| burtim |
Posted on 04/23/08 - 6:08 AM
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I checked the existing prop.. It is the #48-816706A40 that came with the boat. Does this have more blade surface area than others?
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| Binkie |
Posted on 04/23/08 - 7:34 PM
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As a general reference, the rpms should change 200 rpms for each change of pitch. You might have to test a few props before deciding on one. especially if deciding to go with stainless, which will give you your best performance, but are double the price of aluminum. You should try to run your max allowable rpm, with a light load, just you in the boat. Does your motor have t-n-t. and a speedo or gps?. Keep tilting out till speed drops off, then come in slightly.
rich
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| burtim |
Posted on 05/08/08 - 8:01 PM
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I had the chance to try a 10-3/8" x 13 pittch prop (Mercury aluminum Black Max) last weekend. I saw the expected imrovements. I didn't have the opportunity to run the boat at capacity load, but, I was able to trim up and get about 5700 RPM with just me in the boat. I don't want to go to any lower pitch than that since my motor (40 HP Mercury 2 stroke) has a max. WOT recommended at 5,500 RPM. With full fuel, a second person and a little gear, WOT is right at 5,500 and I feel like that will be where I need to stay. Obviously this should improve my boat when at full load- I can't wait to try that.
My motor is mounted in the top holes (the motor is setting at the lowest possible position). It looks like that puts the anti-cavitation plate very slightly above the keel. When I trim up, I get higher RPM, but my boat starts to porpoise. What will happen when I raise the motor to the second or third (highest motor position) holes? Will this make it likely that I could go back to the 14 pitch and get more RPM?
When I finally settle on a pitch through my experimentation and decide to go to a vengeance or Stilletto SS prop., what will happen to the WOT RPM as compared with an aluminum.
I'm having alot of fun playing with my new Whaler 13. I wish I could just go to the lake and forget everything else. Sorry about all the questions but this is the first boat I've ever owned and I want to get her running the best she can before I forget about the technical stuff and get to fun and fishing.
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| Derwd24 |
Posted on 05/08/08 - 9:35 PM
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Burtim,
Give this a read, very good article and illustration on proper engine height.
ARTICLE
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| burtim |
Posted on 06/06/08 - 11:35 AM
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Alright! I moved the motor up to the highest possible position for the Merc 40 horse. I ran it with the 13 pitch prop and put four fat adults in the boat. I can see the AV plate just below the top of the spray at that height. I got good speed and 5200 RPM. It seems like I could trim higher and get even more RPM and speed. When I try that, I get the porpoising others have talked about as we begin to accelerate even more. It seems like the motor is pretty far down when this happens. Shifting everybody forward in my 13' improves things. You can only go so far. Anything else I can do to help the porpoise?
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| alohajim |
Posted on 06/06/08 - 5:05 PM
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Doelfin or Stingray hydrofoil to get on plane more smoothly and at lower speeds. Works well for porpoising.
Mahalo,
jim
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